Overload indicator for electrical apparatus



Dec. 5, 1961 J. c. JOUBLANC OVERLOAD INDICATOR FOR ELECTRICAL APPARATUS2 Sheets-Sheet l Original Filed July V30, 1956 T/ME /N HOURS INVENTOR.J'osep/z C. gfoublarzc BY $420K uizftorn y' 1961 .1. c. JOUBLANC3,012,236

OVERLOAD INDICATOR FOR ELECTRICAL APPARATUS Original Filed July 30, 19562 Sheets-Sheet 2 IN V EN TOR.

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ilnited States Patent 2 ELECTRICAL This case is a division ofapplication Serial No. 600,- 790, filed July 30, 1956, now Patent No.2,886,805, granted May 12, 1959.

This invention pertains to a device for detecting and indicatingoverload conditions in fluid immersed electrical apparatus such asdistribution transformers and in another aspect the invention serves toindicate expiration of the normal life of insulating materials used inthe apparatus.

It is commonly accepted that the expected life of organic insulatingmaterials used in transformers depends to a large extent upon thermalconditions at the hottest spot and that deterioration of such insulationis a function of time, mechanical and dielectric stress as well as thetemperature to which it is subjected. Hence, it is a frequent practiceto provide transformers, particularly, with thermal responsive deviceswhich purport to reflect conditions at the hot spot and to integrate thetotal eiiect of long and short duration overloads regardless of theirmagnitudes.

Such devices may be divided into three broad types. The first includes atemperature sensitive bimetallic element in heat exchange relationshipwith the top transformer oil and in circuit with the windings, in orderto achieve delayed response to injurious long duration overloads ofcertain values and more rapid response to short duration overloads ofgreater values. By giving careful consideration to the physicalcharacteristics of the bimetallic element, namely, its heat input, area,and losses, the bimetal may be designed so that its temperature gradientwith respect to the top oil equals the gradient of the windings withrespect to the top oil. Thus, regardless of ambient temperatureconditions, the temperature of the bimetal will be the sarne as thetemperature of the winding. The amount of load current required to causethe Winding, and accordingly the bimetal, to reach any predeterminedtemperature value, is dependent upon That is, the load current requiredis high for low ambient temperatures, and low current is required forhigh ambient temperatures.

Because transformers of different sizes and ratings have dissimilarthermal characteristics, it is necessary that a bimetallic heatresponsive element be deigned and carefully coordinated with theparticular transformer to which it is applied. Thus, in order to supplythe requirements of a manufacturer producing a complete range ofdistribution transformers, it is necessary to provide a complete- .lyindividual thermal condition indicator for each transformer type andrating, under the limitations of prior art practice. It is obviouslywasteful of design effort, manufacturing facilities and storage spaceand otherwise inconvenient to employ a wide variety of such indicators.

The second broad type of thermal condition indicator consists in atemperature responsive element in the form of either a bimetallicelement or an expansion bulb in heat exchange relation with theinsulating oil and including a separate electric heating coil whichcarries part or all of the transformer load current and is in proximitywith the temperature responsive element for reflecting conditions at thehot spot during large overloads of short duration. Prior art designs ofsuch indicators are Y a transformer I 31112235 Patented Dec. 5, 1961also inflexibly adapted for use with the particular transformer ratingswith which they are coordinated.

The third type of thermal indicator consists in a temperature responsiveelement in direct physical contact with the wire of the transformercoil. This type must be placed in the coil during winding and is usuallylocated in the portion thereof which will reach the highest temperatureunder operating conditions. if the voltage on the winding is high, thetemperature responsive element must be insulated to eliminate a shockhazard. There is always a possibility that failure of the insulationaround the temperature responsive element will result in dangerouspotentials appearing on it. v

A principal object of this invention is to overcome the above notedinconveniences and economic disadvantages by providing a thermaloverload indicator which includes basic components universally adaptableto a wide range of differently rated transformers and also includes lessexpensive interchangeable components for conveniently adapting theindicator to a particular transformer.

Another object is to provide a relatively inexpensive device capable ofdifferentiating between various degrees of overload and emitting acorresponding signal by which the copper temperature ranges due tooverload conditions may be distinguished. One form of the novelindicator is also provided with a counter for totaling the number oflarge magnitude overloads and providing a basis for determining when aninstalled transformer should be replaced by one having a larger currentcarrying capacity.

A further object of the invention is to provide a device which not onlyindicates the temperature ranges to which winding is being subjected butalso anticipates impending dielectric failure of the transformer coilinsulation by emitting a visible or audible signal before actual failurethereof.

More general objects of the invention are to provide an overloadindicator which is inexpensive, durable, compact and easy to install.Other specific objects will appear from time to time throughoutthisspeciflcation.

Generally stated, the novel overload indicator is characterized by abasic universal component, adaptable to a variety of distributiontransformers, and an interchangeable component for achieving adaptationto any particular transformer of given rating. The universal componentconsists in at least two enclosed bimetallic thermally responsiveelements each having a portion of their housing projecting from aresinous or plastic embedment. The entire embedment is installed in thetop transformer oil in heat exchange relationship therewith. Preferably,one of the thermal elements is of the snap acting, single polesinglethrow, manual resetting type and the other is a double pole-double throwsnap acting type. Both thermal elements are connected serially with eachother across the secondary transformer bushings and in circuit with anindicating light exterior to the transformer tank.

The embedment also embraces a flasher which is cut in and out of thesignal light circuit by selectivity of the double throw bimetal. Eachbimetallic element responds to a different critical temperature so thatupon closing the single pole element while the double throw element isin a first position, a steady warning signal is emitted. Whereas, if thetransformer temperature increases until the double throw bimetallicelement snaps to a second position, the flasher is interposed in circuitwith the signal.

-coil assembly 4 are in mutual between the heater and thermal switchesconstitutes the other interchangeable component of the overload indi-'cator.' The barrier is likewise selected in accordance with thermalcharacteristics and ratings of the transformers to which the indicatoris' being applied. The barrier provides a temperature gradient'betweenthe thermal switches and heater so that the said switches will havesubstantially the same temperature gradient with respect to'the top oilasrthe average winding temperature to the top oil temperature. 7

Therefore, the heater element is a thermal image of the winding hot spottemperature; whereas, the thermal switch is a thermal image of theaverage winding tem perature. This difference between the hot spottemperature and average winding temperature is usually in the order of10 C. during normal operating conditions.

Those versed in the art will appreciate that the invention enablesincorporation of two variables into the thermal responsive component.may be varied by changing the heater resistance, and the response rateof the bimetals may be varied by changing the insulation barrier betweenthe heater and bimetals. A more detailed description of the inventionwill be set forth connection with the following drawings in which:

FIG. 1 is an elevational view, with parts broken away, showing atransformer embodying the invention;

FIG. 2 is an enlarged side elevational view of the overload indicatorconstituting the invention;

FIG. 3 is a front elevational view of the overload indicator;

FIG. 4 is a top view partly in section taken on a line correspondingwith 44 of FIG. 3 and looking in the direction of the arrows;

FIG. 5 is a graph depicting the operating characteristic of theinvention; and

7 FIG. 6 is a schematic representation of the inventio associated with atransformer. i

In FIG. 1, the novel overload indicator. is designated generally by thereference numeral 1 and shown immersed in an insulating fluid such asoil partly filling a transformer tank 2 to a level indicated by thedashed lines 3. A transformer core and coil assembly 4, whose thermalcondition the invention is intended to indicate, is also under oilwithin the tank. Indicator 1 and core and heat exchange relation throughthe oil medium. a

Before entering into a detailed discussion of overload indicator 1,reference is made to FIG. 6 where the indicator and the equipment withwhich it is associated are shown diagrammatically for convenientconsideration. Transformer 4 is a well known dual voltage secondary typeof distribution transformer having secondary leads 5 connected for threewire service, supplying an electrical load through tank insulatingbushings 6. One of the lead wires 5 has connected in it a heater element7 associated with indicator 1. In proximity with heater 7 and also beingpart of the indicator is a pair of bimetallic thermostatic switches 8and 9. Switch 8 is a single pole, single throw, manually resettable,snap acting, temperature responsive switch, hereinafter called aS.P.S.T. switch.

Switch 9 is a single pole, double throw, snap acting, self resettingtemperature responsive switch, hereinafter called a S.P.D.T. switch.Switches 8 and 9 are serially connected in a signal circuit to bedescribed.

In order to obtain the most satisfactory indication of overloadconditions in a transformer connected for three rvire service, see FIG.6, where in some instances over- That is, its heat input load currentmay be supplied through lines X and X or X and X only, a second set ofbimetallic elements 8, 9, a heaterelement 7, and a flasher f and counterunit 0 if the latter is desired, must be insertedin the secondary lead 5which is connected to bushing X 1 Since the suggested circuit issymmetrical with that of FIG. 6 it is not shown, for the sake ofbrevity, but it is deemed readily understandable by those versed in theart.

Interchange of heat between switches 8, 9 and the transformer oil andheater 7 is controlled by a readily interchangeable thermal barrier 16interposed between them. The thermal characteristics of insulatingbarrier 10 will determine the rate at which the thermal elements inswitches 8 and 9 will reach the temperature value of the heater element7. According to conventional analysis, this rate is dependent upon thebarriers coefficient of conductivity K, its area, thickness and thetemperaturedifference over the thickness of barrier 10. Where-K isexpressed in calories per second per degree centigradeper centimeter,the rate of heat transfer through barrier 10 will be equal to Kdt/dsA,where A is area and the differential is that of temperature differencewith respect to time. Hence, the thermal lag of bimetal switches 8, 9with respect to heater 7 canbe changed to meet requirements of aparticular transformer design by merely chang ing the character of thebarrier material 10 as indicated, Expressed in another way, the novelindicator can dupl-i cate the gradient of a large number of transformercoil temperature gradients, by changing, in eflect, the heat transfercoeflicient of the barrier and/ or the heat output of the heater elementby changing resistivity of the latter.

The signal circuit commences at the left hand bushing 6 in FIG. 6- witha low voltage supply lead 11 in the form of a metal embedment supportingbar connected to the series switches 8, 9. A pair of wires connect toalternate terminals of S.P.D.T. switch 9 and include a counter c, whichis optional, and an intermittent switch such as flasher f.- Output leadsof the counter and flasher join in a common wire 12 extending to asignal light 13 which is located exterior to transformer tank 2 andvisible a considerable distance therefrom. Signal light 13 connects tothe middle low voltage bushing 6 by means of a lead Wire14. V

In FIG. 6, it may be assumed that the transformer 4 is conducting atolerably safe load whereupon its oil temperature rise is such thatswitch 8 is open and S.P.D.T. switch 9 is closed in the first of itsselective positions. Under these conditions signal light 13 isdeenergized or dark. If the load is increased so that the temperature ofthe transformer copper rises to a value which increases deterioration ofthe transformer insulation, then S.P.S.T.

switch 8 closesand signal light 13 emits a steady glow.

This condition is indicated graphically on the typical coppertemperature vs. time with varying load curve of FIG. 5 at point a. Ifthe temperature of the transformer drops to a safe value, however,switch 8 will remain closed until manually opened by means of a resetbutton 15 reachable from the exterior of the transformer tank 2.

If the overload continues to increase until the transformer temperaturereaches a critically injurious value, switch 8 remains locked in andS.P.D.T. switch 9 snaps to its alternate position in response to theincreased temperature, whereupon it interposes flasher j in series withsignal light 13. The light then emits a more conspicuous intermittentsignal announcing that the transformer is critically overloaded.

The last described condition is indicated graphically in FIG. 5 at thepoint b on the curve. If the critical overload continues fora period oftime and then subsides to a level below point 0, S.P.D.T. switch 9 'willsnap back to its first position and eliminate the flasher from the light13 circuit. Light 13 will then continue to emit a steady glow untilreset by button 15.

Counter c is an optional feature of the novel indicator. It may belocated exterior to transformer tank 2 for conveniently checking thetotal number of seriously injurious overloads which the transformer hasundergone, so its use is recommended. The counter. may be anycommercially available type which advances one numeral each time it isenergized; As employed here it would advance when the transformer iscooling, which means when S.P.D.T. temperature responsive switch snapsback to its position in FIG. 6. V i

The inventive overload indicator may also be provided with means forsubjecting a sample piece of transformer insulation to thermal,mechanical and dielectric stresses comparable with those to which theactual transformer insulating material is subjected. Explaining thisattribute of the invention will'be deferred until later.

Attention is now invited to FIGS. 2, 3, and 4 showing the specificcharacter of the indicator. Note that switches 8, 9, flasher f, strip 11and push button 15 together constituting the universal component of theindicator 1, are embedded in a solid prism of resinous material 20. Leadstrip 11 is preferably of copper and not only performs as part of thesignal circuit but aifords a very convenient means for supporting theindicator from studded bushing 6, no special mounting means beingrequired.

The interchangeable components of indicator 1, namely, I

heater strip 7 and thermal insulation barrier 10, are secured againstswitches 8 and '9 by means of upper and lower clamps 21 and 22, whichare held by knurled studs 23 cast firmly in resinous embedment 20.

Heater strip 7 may be made of a relatively poorly con ductive materialsuch as stainless steel or in thinner strip of more conductive materialsuch as copper. In any case, heater 7 will be designed so that itsheating capacity and output will bear a proportionality to heatingconditions in the vicinity of the transformers hottest spot. In thisillustrative embodiment, heater strip 7 is formed with shoulders 2interposed closely between clamps 21 and 22 so that the strip cannotslip vertically. One end of heater 7 is conveniently connected to asecondary bushing 6 by a lugged pigtail and the other end is connecteddirectly to a secondary winding lead wire 5. It will also be noted thatone of the broader surfaces of heater 7 is in direct heat exchangerelationship with the top transformer oil and the other is in the samerelationship with thermal insulating barrier 10.

Thermal barrier 10 may have -a configuration similar to that of heater 7except that no extension for connecting lead wires is necessary. Sincethe barrier 10 controls the rate of heat flow from the top oil andheater 7 to the thermal switches 8 and 9, it may be composed oftemperature resistant plastic, glass cloth, insulating paper, the topoil itself or a combination of these elements. Although it is asignificant attribute of this disclosure that the heater 7 and thermalbarrier 10 be readily interchangeable with respect to the basiccomponents carried by the embedment 20, it is also within the scope ofthe invention to embed either or both the barrier and heater.

As explained earlier, the fundamental idea of an overload indicator suchas that under consideration is to reflect conditions at the transformerhot spot and to account for the fact that the allowable overloadmagnitude is a function of the time during which it is applied. Duringperiods of normal transformer operation a definite temperature gradientbetween the hot spot and top oil will be established at which timeswitches 8 and 9 do not close the signal circuit. However, if thetransformer is suddenly called upon to deliver an increased load, thehot spot and heater 7 temperatures will increase very rapidly, but thetemperature of the top oil will lag behind. By selecting an insulatingbarrier 10 having proper heat transfer characteristics, the timeconstant of the bimetallic switches 8 and 9 will be such that they willattain their response temperatures after a time delay, if they everattain it at all. Under these circumstances the top oil temperaturecontinues to rise exponentially and ultimately causes switches 8 and 9to respond in order and energize 6 signal lamp 13. If the overload dropsoff in a period of time not exceeding that in which the transformerinsulation life loss becomes excessive, the switches will never attaintheir operating temperatures. The temperature of switches 8 and 9 willthen decline in accordance with the hot spot temperature. The aforegoingsequence of events is illustrated graphically in FIG. 5 where the pointa onthe curve indicates the temperature conditions under which switch 8causes a steady light to be emitted by signal 13, point b the conditionwhere switch 9 closes to cause an intermittent signal to be emitted, andpoint 0 the condition where switch 9 opens the flasher circuit andcauses the counter c to operate. As explained earlier, it is desirable,though not indispensable, that switch 8 controlling the steady signallight be manually reset to open position by means such as push button15.

Further reference to FIG. 2 reveals that reset button 15 may extend fromthermal switch 8 outward of tank 2 through any conventional fluidsealing gland like 17 so that button 15 may be reached exteriorly of thetank. Obviously, it is within the ability of a skilled artisan toactuate the reset button by means of linkage, not shown, passing out oftank 2 above oil level 3.

Plastic embedment 20 may also be modified by inclusion of holes which donot unduly weaken it, yet allow oil to circulate therethrough. In thismanner, the thermal characteristics of the indicator may be further a1-tered.

The novel overload indicator also features mean for indicating impendingdielectric failure of the organic insulating materials of thetransformer. This is accomplished by subjecting a properly chosen samplepiece of insulating material to the same dielectric and thermal stressesper unit area as comparable insulation at the hot spot. For this purposea sample piece of insulation 27, see FIG. 2, is held on an offsetportion 7a of heater strip 7. Sample 27 is exposed to the heatinginfluence of strip 7 and the top oil and is held in place by a U-shapedspring clip 28 of conductive material such as beryllium'copper. Apressure finger 29 bears on spring clip 28 and is anchored on a terminalbolt 30 where it connects to a lead wire 31 from a warning buzzer coil32, see FIG. 6. The opposite side of the buzzer coil 32 connects to oneside of the transformer secondary by means of lead 14' joined to abushing 6. Hence, spring clip 28 acts as an electrode supplied by onesecondary line and heater coil 7 acts as another electrode by virtue ofit being connected to an opposite secondary line through pigtail 25. Thetwo electrodes thus subject sample insulation 27 to substantially thesame potential stress as is the insulation between the layers of thetransformer winding. When sample 27 deteriorates to such extent that itis incapable of insulating properly, it will break down electrically soas to complete the circuit through warning buzzer 32. As a result ofproper coordination, this gives an audible sig- Eal dthat failure of thetransformer insulation is near at Lead 31, illustrated in this preferredembodiment as a dashed line leading to the buzzer coil 32, may also beconnected directly to a terminal of signal light 13 if it is desired toeliminate buzzer 32. With this circuit it is possible to determine ifsignal light 13 is lit by closure of thermal switches 8 and 9 due tooverload or by rupture of insulation sample 27 due to dielectric failureby depressing reset button 15. If reset button 15 does not extinguishlight 13 it may be assumed that failure of the insulation sample,warning of impending transformer insulation failure, is the cause of thesignal.

If the potential between beryllium spring clip 28 and heater 7 is, forexample, volts, then if sample insulation 27 were .005 inch thick, itwould be dielectrically stressed to 24 volts per mil. Preferably, thiswould be a little greater stress than that applied to the actualtransformer insulation so that the sample would break down the heatingelement 8 can be made to simulate coilconditions. q

Since different manufacturers stress their transformer h insulationdielectfically through a. range of approximately to50'volts per mil.insulation thickness, it is obvious I The invention also simulates themechanical stress to which the transformer winding insulation issubjected by 'means of the bearing pressure exerted onthe sampleinsulation by the spring clip 28. Thus the invention closely approachesactual conditions which combine .to cause insulation failure'in atransformer, that is, by simulating the electrical stress and themechanical stresswhich the insulation undergoes in the transformerwinding.

The immediately aforegoing feature of this invention is an improvementover US. Patent No, 2,457,879, assigned to the assignee of the instantinvention, disclosing an insulation age indicator which reliespurelyupon mechanically stressing a sample piece of insulation subjectedonly to the temperature and deteriorating influence of the transformeroil in which it is immersed.

I Although only a preferred embodiment of the invention has been'shownfor facilitating description of its general features, the disclosure isto be construed as illustrative rather than limiting for the inventionmay be variously embodied and is to be interpretedas claimed.

It is claimed:

1. In combination, a load and insulation condition indicator for atransformer immersed in dielectric fluid, a

*pair of transformer secondary terminals, said load indicator includinga heater element in circuit with a secolndary winding of saidtransformer and one of said terminals, an insulation sample meansimultaneously ex posed to the heat of said element and to said fluidand to the electrical potential between terminals, said heater elementgenerating heat in accordancewith the secondary current in saidtransformer and establishing a tempera- 'ture gradient with respect tosaid dielectric fluid proportional to the gradient between thetransformer hot spot and said dielectric fluid, signal means connectedbetween the other of said terminal and said one terminal and normallydeenergized by said insulation sample means, whereby dielectric failureof said sample means will indicate substantial expiration of saidtransformer insulation by passing suflicient leakage current to energizesaid signal means. 7

2. In combinatioma load andinsulation condition indicator foratransformer immersed in a dielectric fluid, a pair of transformersecondary terminals, 'said load indicator including a heater element incircuit'with a secondary winding of said transformer and one of saidterminals, an insulation sample disposed between two-electrodes andsimultaneously exposed to the heat o-fsaid element and to said fluid,said heater element generating heat in accordance with the secondarycurrent in said transformer and establishing a temperature gradient withrespect to said dielectric fluid proportional to the gradient betweenthe transformer hot spot and said dielectric fluid,

means'for applying an electrical potential to said electrodes equal tothat which exists between, said terminals,

one of said electrodes comprising spring clip means which subjects saidinsulation sample to the mechanical stress signal means.

References Cited in the file of this patent UNITED STATES PATENTS2,175,893 Hill OctflO, 1939 2,313,975 Spear et a1 Mar. 16, 19432,457,879 Earle Jan. 4, 1949 2,567,921 Boehm Sept. 18, 1951 OTHERREFERENCES Satterlee:

AIEE Technical paper 52-88, 11 pages, January 1952.

